With the curtain set to rise on the blockbuster Sean Bell police-shooting trial, some observers wonder whether hope for the three accused officers may come from the very grand jury that indicted them.

The case will hinge on whether or not the charged cops acted reasonably when they unleashed a fusillade of 50 bullets on Bell, 23, and his two friends, who were unarmed, as they left Bell’s Queens bachelor party on his wedding day.

According to court papers, at least some of those 50 shots have already been deemed to be legitimate.

“The grand jury found the first few shots from all of the police officers to be justified,” and therefore two officers on the scene were not charged at all, “but that justification quickly evaporated, leaving Detective Oliver and Detective Isnora chargeable with homicide,” the court papers state.

Sources close to the probe say it’s possible that the judge trying the case will find some justifications for some of the shots, too.

“When did the shots become unjustified?” is the pivotal question, said a law-enforcement source familiar with the investigation. “It took nine to 12 seconds. It’s absurd. If you fire four shots, OK, but five you’re not [justified]?”

Police union leaders from the metropolitan area, and some from as far away as Boston, will show up in force to back the detectives because of the chilling effect a conviction might have on their work.

“The impact of the case will force everyone to reassess what they do because this was a tragedy, not a crime,” said Detectives Endowment Association president Michael Palladino.

The testimony is to begin with a retelling of the night of Nov. 25, 2006, when Bell and his pals, Trent Benefield and Joseph Guzman, were out at Club Kalua, a strip joint in Jamaica, Queens.

The club was a troubled place where neighbors complained about women performing oral sex on men in the streets, sources said.

Those troubles brought a group of undercover officers to the club that evening.

It was to be the team’s last night at the club, and they needed just one more arrest to justify shutting it down under a public-nuisance law, said another source.

But as Bell and his friends were there celebrating, at one point he and his crew stepped outside and noticed a young woman standing near a black Cadillac Escalade. They solicited her for sex, according to sources.

“I’ll do two of you, but I won’t do you all,” said the young woman, according to sources.

And that’s when an argument broke out between the group of men and a man inside the SUV.

Officer Isnora said he heard Guzman yell out he was going to “get my gun,” with Bell adding, “Let’s f – – – him up.”

A source added that one of Bell’s friends corroborates the cop’s version of events, telling the grand jury he heard someone yell out “gun.”

Bell and his friends got in their car. Some of Bell’s friends also testified to the grand jury that they saw Isnora stand directly in front of the 23-year-old man’s car.

They say they witnessed the cop getting bumped when Bell accelerated, said a source.

That’s when shots rang out.

Oliver fired 31 times, reloading once. It was his shots that killed Bell.

Isnora was the first to fire, and got off 11 rounds. Both are charged with manslaughter and face up to 25 years in prison if convicted.

Cooper, who fired four times, is charged with reckless endangerment. One of his bullets hit the nearby Jamaica AirTrain station.

It will be up to Queens Supreme Court Justice Arthur Cooperman to decide the fate of the cops. The defense opted out of a jury trial after the Appellate Division denied a change of venue prior to jury selection.